1. Centralized Base Station Having the Structure Based on Remote Radio Frequency Units
In the mobile communication system, the transmission, reception and processing of the wireless signals are performed by base stations (BTS). As shown in FIG. 1, a conventional BTS 10 is mainly composed by a baseband processing subsystem 11, a radio frequency (RF) subsystem 12 and antennas 13, and one BTS may cover different cells (cell 14) through a plurality of RF antennas. FIG. 2 presents another kind of base station using distributed transmission sources, i.e., using the system architecture of a centralized base station based on radio frequency units. As compared to the conventional base station, such a centralized base station based on radio frequency units has many advantages: allowing to replace one macro cell based on the conventional base station with a plurality of micro cells, thereby best accommodating different wireless environments and increasing wireless performances such as capacity, coverage and etc. of the system; The centralized structure makes it possible to perform the soft handoff in the conventional base station by a softer handoff, thereby obtaining an additional process gain. The centralized structure also makes it possible to use costly baseband signal processing resources as a resource pool shared by a plurality of cells, thereby obtaining benefits of commonly sharing and reduced system cost. More details of this technique are disclosed in PCT patent WO9005432 “Communications system”, U.S. Pat. No. 5,657,374 “Cellular system with centralized base stations and distributed antenna units”, U.S. Pat. No. 6,324,391 “Cellular communication with centralized control and signal processing”, China patent application CN1464666 “Soft base station system based on fiber optic stretch and synchronous method thereof”, China patent application CN1471331 “Base station system for mobile communication” and United States Patent application US20030171118 “Cellular radio transmission apparatus and cellular radio transmission method”.
As shown in FIG. 2, the centralized base station system 20 based on remote radio frequency units is composed of a centralizedly configured central channel processing subsystem 21 and remote radio frequency units 24 which are connected through the wideband transmission link or network. The central channel processing subsystem mainly comprises functional units such as the channel processing resource pool 22, the signal routing distribution unit 23 and etc., wherein the channel processing resource pool is formed by stacking a plurality of channel processing units, and performs tasks such as baseband signal processing, and the signal distribution unit dynamically allocates channel processing resources according to the traffic of different cells to realize effective sharing of the processing resources among multiple cells. Besides the implementation inside the centralized base station as shown in FIG. 2, the signal routing distribution unit may also be implemented as a separate device outside the centralized base station. The remote antenna element is mainly constituted by functional units such as the transmission channel's radio frequency power amplifier, the reception channel's low noise amplifier, antennas and etc. The link between the central channel processing subsystem and the remote antenna unit may adopt transmission medium such as optical fiber, coaxial cable, microwave and etc.; the signal transmission may be done by way of digital signals after sampling, or analog signals after modulating; the signals may be baseband signals, intermediate frequency signals or radio-frequency signals.
However, the micro cell structure of the centralized base station system based on remote radio frequency units may cause the problem of frequent handoff. To overcome the problem, the inventors propose an effective solution in a patent application entitled “micro cell management method in the mobile communication system using the centralized base station”, wherein dynamic cell control is performed for the cells under coverage by the base station according to the parameters such as the UE's moving speed, cell load conditions, processing resource occupation of the centralized base station. Such a dynamic cell control is to dynamically group a plurality of geographically adjacent cells with the similar parameters into one cell. For this dynamically generated cell, the downlink scramble code is the same, and the radio remote frequency units corresponding to the original micro cells forming the dynamic generated cell constitute a distributed radio frequency transceiver system of the dynamically generated cell. In addition, according to the patent application, it is also possible to employ a fixed configuration method to merge neighbouring micro cells into one cell, i.e., to fixedly configure the geographically adjacent micro cells in some areas into one cell according to a predetermined system configuration. This is mainly suitable for the case where system design capacity is small at time of initial network construction. For the convenience of explanation, such a cell formed by dynamically or fixedly merging the geographically adjacent micro cells is called a complex cell.
Although the above improvement has been proposed, the inventors keep on seeking new improvements. The inventors recognize that when several micro cells are merged into a complex cell, since the code resources (that is, channel resource) do not increase and the cell's size increases, the channel capacity is relatively insufficient as compared to the original micro cells.
The inventor further find that since a complex cell is formed by more than one micro cells, there is certainly more than one antennas, and thus it is possible to employ multiple antenna transmitting/receiving (MIMO) technique in the complex cell to increase communication capacity, thereby remitting or overcoming the adverse effect.
2. Multiple Antenna Transmitting/Receiving (MIMO) Technique
The multiple antenna transmitting/receiving (MIMO) technique is a new technique recently developed for effectively increase spectral efficiency. In the present standardization work of 3GPP (third generation cooperation project) on UMTS (universal mobile communication system), researching is also performed with respect to this technique. For MIMO technique and its application in UMTS, please refer to literatures such as “From theory to practice: an overview of MIMO space-time coded wireless systems, IEEE Journal on Selected Areas in Communications, vol. 21, no. 3, April 2003”, 3GPP work document “R2-010504, Overview of Multiple-Input Multiple-Output Techniques for HSDPA” and etc. There are mainly two kinds of MIMO techniques at present, one is based on multiple antenna transmission diversity and reception diversity for maximizing the diversity gain, another is based on channel code reusing scheme for maximizing the data rate, wherein the MIMO based on channel code reusing scheme is most representative.
FIG. 3 shows a structure 30 of a MIMO system transmitting terminal based on channel code reusing scheme in a multi-code system such as HSDPA (high speed downlink packet access) and etc. The high speed data stream after channel encoding branches into M·N substreams through a branching unit 31, wherein M is the number of antennas of transmitting terminals, and N is the number of parallel downlink code channels in the multi-code system such as HSDPA and etc. Each data stream group constituted by M substreams is spread through a corresponding downlink channel code respectively in a spreading unit 32. M signals are then synthesized, appended with dedicated pilot sequences orthogonal to each other and transmitted through M antennas respectively. It can be seen that since each group of M substreams reuses one downlink channel code, the data rate is increased by a factor of M.
In the prior art, the antennas of a transmitting terminal and a receiving terminal in the MIMO system are centralizedly located. As shown in FIG. 4, to reduce the correlation between antennas as far as possible, it is usually needed to guarantee that the spacing between antennas is at least above a half wavelength. Although the antennas of the transmitting and receiving terminals are apart from a certain given distance, since the distance between the base station and the mobile terminal is relatively large, the existing MIMO system is suitable for the ideal communication channels meeting the following conditions in the downlink direction:
(1) the multipath numbers and multipath delays from different transmitting antennas to any receiving antenna are equal;
(2) the average path losses from different transmitting antennas to any receiving antenna are equal;
(3) the multipath channel fading of propagation paths from different transmitting antennas to any receiving antenna are mutually independent;
(4) the interferences and the noise power spectra received by different receiving antennas are equal and independent from each other.
Under the above ideal channel condition, if it is assumed that the receiving antenna number is P (P≧M), and the multipath number of the MIMO channel is L, the multipath channel vector from the m-th transmitting antenna to the p-th receiving antenna is:hm,p=(hm,p,1, hm,p,2, . . . hm,p,L)T  (1)
The estimation of its channel parameter may be obtained by using a dedicated pilot sequence. If using the code reusing scheme, the signal vector of M substreams spread by the k-th channel code and transmitted through M transmitting antennas is:xk=(xk,1, xk,2, . . . xk,M)T  (2)
FIG. 5 provides a functional block diagram of a MIMO receiver 40 based on V-BLAST detector according to the prior art. As shown in FIG. 5, after performing multipath tracking and despreading on all the (L) multipath components using the k-th channel code through a multipath tracking and despreading unit 41, the signal vector of reception signals of the p-th receiving antenna is:yk,p=(yk,p,1, yk,p,2, . . . yk,p,L)T  (3)
Let the L×M multipath channel matrix of the p-th receiving antenna be Hp=└h1,p, h2,p, . . . hM,p┘, thenyk,p=FkHpxk+vk  (4)
Wherein vk is a noise vector, Fk is a L×L code correlation matrix determined by the autocorrelation characteristic, after downlink scrambling, of channels corresponding to the k-th channel code. By using the above equation, the processing of time-space RAKE merging unit 43 as shown in FIG. 5 can be expressed as:
                              z          k                =                                            ∑                              p                =                1                            P                        ⁢                                          H                P                H                            ⁢                              y                                  k                  ,                  p                                                              =                                                    R                k                            ⁢                              x                k                                      +                          n              k                                                          (        5        )            
Wherein zk=(zk,1, zk,2, . . . , zk,M)T is the time-space RAKE merging output corresponding to signals of M substreams of channels which corresponding to the k-th channel code, nk is the noise component contained in the output, and Rk is a code channel correlation matrix corresponding to the k-th channel code:
                              R          k                =                              ∑                          p              =              1                        P                    ⁢                      (                                          H                p                H                            ⁢                              F                k                            ⁢                              H                p                                      )                                              (        6        )            
In practice, in the received signals containing symbol xk,j transmitted by transmitting antenna j, there may further possibly existed two kinds of path components: One is the path components having the same delay but received by different receiving antennas, another is the multipath components received by the same receiving antenna but having different delays. There is the spacial interference caused by channel code reusing in these path components, and the multipath interference caused by the incomplete orthogonality between the downlink scramble code and its delayed duplication. As will be readily seen, the essence of the time-space RAKE receiving process as shown in equation (5) is the max ratio merging of all of the above spatial domain and time domain path components corresponding to the symbol xk,j transmitted by each transmitting antenna. The processing of V-BLAST detector 44 after time-space RAKE processing is to solve minimum mean square error (MMSE) solution vector of xk based on equation (5). If neglecting the non-orthogonality between multipaths and approximating Fk as a identity matrix, the V-BLAST is equivalent to a multiuser detector only for canceling spacial interference. Contrarily, it is equivalent to a multiuser detector for canceling the spacial interference and multipath interference at the same time.
More details about the MIMO system based on channel code reusing technique and the receiver based on time-space RAKE receiving and V-BLAST detector can be found in U.S. Pat. No. 6,370,129, “High-speed data services using multiple transmit antennas”, “Performance of space-time coding for 3GPP HSDPA service under flat and frequency selective fading conditions”, International Conference on 3G Mobile Communications Technologies, 2002 and etc.
However, as detailedly described in the following, in the context of complex cell as mentioned herein, if using MIMO technique, the channel condition is different from the ideal channel condition of the MIMO system of the prior art, wherein since the different geographic positions of transmitting antennas of the transmitting terminal, the spatial distance and propagation path from each of the transmitting terminal's transmitting antenna to the mobile terminal is different, and thus the average path loss from a different transmitting antenna to the mobile terminal's receiving antenna is different. The multipath number and corresponding delay from a different transmitting antenna of the transmitting terminal to any receiving antenna of the mobile terminal are different. Therefore, it is impossible to use existing MIMO technique in the complex cell.
Therefore, it is desired to provide a centralized base station system based on remote radio frequency units and the method thereof, wherein it is able to enforce the MIMO technique matching the channel characteristic of complex cell in the complex cell.